In fiber optics generally the term cladding mode stripper refers to any method whereby light propagating in the cladding (i.e. not the core in which waveguide modes are desired) is caused to propagate out of the cladding. The cladding mode stripper must absorb or redirect the stripped light. Most single-mode fiber for use in telecommunications applications includes a high index core, a slightly lower index cladding, and a high index polymer (substantially higher than the cladding index) coating on the outside for protection. The high index polymer also functions as a cladding mode stripper.
In high power fiber lasers (HPFLs), however, the cladding mode stripping becomes more difficult as it must perform at a substantially higher power level. Fluorescence and escaped core light from high-power splices will generally propagate in the cladding of a HPFL. In contrast to telecom fiber, a HPFL has a low index rather than high index coating (in order to propagate the low-brightness pump light). Such light will need to be stripped. Also, the output of any fiber laser is eventually a glass/air interface, leading to a 3.5% Fresnel reflection that will propagate backwards into the cladding and will also need to be stripped. In kilowatt-class systems this is a large amount of power, comparable in magnitude to unused pump power, which is the primary reason for the cladding mode stripper.
In HPFLs based on three-level gain media (e.g. Erbium, Ytterbium) the length of the gain fiber is typically restricted to absorbing only 95-97% of the pump light; if the fiber gets much longer then the end of the fiber will have very weakly pumped regions with appreciable ground state absorption.
Ground state absorption acts as a loss mechanism for the signal propagating through the end of the fiber. In oscillator configurations such loss leads to poor slope efficiency due to large cavity losses, and in Master Oscillator/Power Amplifier [MOPA] configurations this leads to poor efficiency due to the reabsorption of the signal and likely additional fluorescence from the excited state caused by this reabsorption. Furthermore, this weakly pumped absorbing region may act as a saturable absorber (an absorber which, given enough energy, will suddenly become transparent, because the ground state has been depleted). At low powers the weakly pumped region may be a mere loss mechanism, but at high powers very dangerous effects can happen due to this saturable absorber effect within the unpumped region.
In co-pumped MOPA configurations a weak signal is typically incident on the fiber at the same point as the full pump power (i.e. no pump has been absorbed yet). This leads to a large population inversion at the beginning of the gain fiber, and, due to the long upper state lifetime in Erbium and Ytterbium fibers, leads to good energy storage. Unfortunately, an energy storage medium at one end, with a saturable absorber at the other end, coupled with parasitic reflections that are present in almost any fiber-based system, defines the most general type of passively Q-switched laser. In high power CW MOPA systems passive Q-switching effects are undesired and can lead to pulse energies well beyond the damage threshold of glass, destroying the laser and its components. If dichroic filter protection is not used on the pump diodes, it can also destroy the very expensive pump diodes. It is therefore very important to avoid passive Q-switching in high power CW fiber MOPAs, and keeping the fiber short enough to avoid weakly pumped regions is part of this strategy, along with eliminating parasitic reflections as much as possible.
Unfortunately, keeping the fiber short implies that some pump power will not be used and will continue propagating beyond the end of the gain fiber. This pump power must be stripped to avoid degrading the beam quality of the output or possibly harming other components downstream from the gain fiber. In practice what makes this unused pump power management so difficult is that the pump absorption rate (dB/meter) can vary with pump current and hence pump power. This occurs, for example, in the case of pumping ytterbium-doped fiber at 976 nm, as the Ytterbium absorption peak at 976 nm is very sharp and narrow and small pump wavelength drifts with diode current can cause the absorption rate to change dramatically. Thus, if one designs a MOPA for 13 dB pump absorption at maximum pump current, on the way up to the maximum pump current the absorption rate might be slightly lower at intermediate pump currents, leading to greater than expected unused pump power. In kilowatt-class systems, unused pump light can reach into the hundreds of watts due to these effects.